Receiver for receiving data in a broadcast system using redundancy data

ABSTRACT

A receiver for receiving data in a broadcast system includes a broadcast receiver that receives, via the broadcast system, a receiver input data stream including plural channel symbols represented by constellation points in a constellation diagram. A demodulator demodulates the channel symbols into codewords and a decoder decodes the codewords into output data words. A broadband receiver obtains redundancy data via a broadband system, the redundancy data for a channel symbol including one or more least robust bits of the channel symbol or a constellation subset identifier indicating a subset of constellation points including the constellation point representing the channel symbol. The demodulator and/or the decoder is configured to use the redundancy data to demodulate the respective channel symbol and to decode the respective codeword, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/723,537 filed Oct. 3, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/422,919 filed Feb. 20, 2015, which is a NationalPhase of PCT/EP2013/074363 filed Nov. 21, 2013, and claims priority toEuropean Patent Application No. 12194676.8 filed Nov. 28, 2012. Theentire contents of each of which are incorporated herein by reference.

BACKGROUND Field of the Disclosure

The present disclosure relates to receiver and a corresponding receivingmethod for receiving data in a broadcast system. Further, the presentdisclosure relates to a broadcast system.

The present invention relates, for instance, to the field of DigitalVideo Broadcasting (DVB) utilizing Orthogonal Frequency DivisionMultiplexing (OFDM). Further, the present invention can be applied inother systems, such as a DAB (Digital Audio Broadcasting), DRM,MediaFlo, ISDB, ATSC (e.g. 3.0) or LTE broadcast system.

Description of Related Art

The transmission parameters of known broadcast systems, such as thebroadcast systems in accordance with the DVB-T2 standard (secondgeneration digital terrestrial television broadcast systems standard),are generally optimized for fixed reception with stationary receivers,e.g. with roof-top antennas. In future broadcast systems, such as theupcoming DVB-NGH (DVB Next Generation Handheld; in the following alsoreferred to as NGH) standard, a mobile receiver (which is the main focusof this upcoming standard) shall be enabled to receive data correctlyalso in bad reception situations, e.g. despite suffering from multipathpropagation, fading effects and Doppler shifts. Such broadcast systemsare particularly characterized by the fact that there is generally nofeedback channel and no signalling from receivers to transmitters.

A receiver for receiving data in a broadcast system by which theprobability of error-free reception/reconstruction of data by a mobilereceiver is increased compared to receivers in known broadcast systems,even under bad reception conditions, is disclosed in WO 2011/080020 A1.The disclosed receiver comprises

a broadcast receiver unit for receiving from said broadcast system areceiver input data stream segmented into frames, wherein basic codewordportions of codewords are mapped onto said frames, a codeword comprisingsaid at least a basic codeword portion generated from an input data wordaccording to a first code,

a data demapper for demapping the basic codeword portions from saidframes of the receiver input data stream,

a decoder for error correction code decoding said codewords into outputdata words of at least one output data stream in a regular decoding stepby use of the basic codeword portion comprised in a codeword,

a check unit for checking if the regular decoding of a codeword iserroneous,

a unicast request unit for requesting, if said regular decoding of acodeword is erroneous, through a unicast system an auxiliary codewordportion of the erroneously decoded codeword for use as incrementalredundancy in an additional decoding step,

a unicast receiver unit for receiving from said unicast system anauxiliary codeword portion of the erroneously decoded codeword,

wherein said decoder is adapted to decode the respective codeword againin an additional decoding step by additionally using the receivedauxiliary codeword portion, and

a data output for outputting said at least one receiver output datastream segmented into said decoded output data words.

The main use of redundancy data is the increase of the coverage area forterrestrial broadcasting. Subscribers located at the edge of thecoverage area of a broadcast system (also called broadcast network) aresuffering from low receptions levels, which may hinder error-freedecoding. This is also true for indoor reception or if large objectsattenuate the transmitted signal. To counter this problem theutilization of a (wired or wireless) broadband system (also calledbroadband network) for providing additional redundancy for enablingerror-free reception has been proposed. In many cases only a few dBsreceived signal level are missing for the correct demodulation anddecoding of the broadcast data, resulting in an additional redundancydata stream of few hundred kbit/s. Furthermore other channel impairmentslike burst noise or narrowband interferer create decoding errors in asheer broadcast reception scenario which are corrected with theadditional redundancy data stream.

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventor(s), to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

SUMMARY

It is an object to provide a receiver for receiving data in a broadcastsystem using redundancy data for further increasing the probability oferror-free reception/reconstruction of broadcast data. It is a furtherobject to provide a corresponding receiving method, a broadcast system,as well as a corresponding computer program for implementing saidreceiving method and a non-transitory computer-readable recording mediumfor implementing receiving method.

According to an aspect there is provided a receiver for receiving datain a broadcast system comprising:

a broadcast receiver that receives, via said broadcast system, areceiver input data stream comprising a plurality of channel symbolsrepresented by constellation points in a constellation diagram,

a demodulator that demodulates said channel symbols into codewords,

a decoder that decodes said codewords into output data words,

a broadband receiver that obtains redundancy data via a broadbandsystem, said redundancy data for a channel symbol including one or moreleast robust bits of the channel symbol or a constellation subsetidentifier indicating a subset of constellation points including theconstellation point representing the channel symbol,

wherein said demodulator and/or said decoder is configured to use saidredundancy data to demodulate the respective channel symbol and todecode the respective codeword, respectively.

According to a further aspect there is provided a correspondingreceiving method for receiving data in a broadcast system comprising:

receiving, via said broadcast system, a receiver input data streamcomprising a plurality of channel symbols represented by constellationpoints in a constellation diagram,

demodulating said channel symbols into codewords,

decoding said codewords into output data words,

obtaining redundancy data via a broadband system, said redundancy datafor a channel symbol including one or more least robust bits of thechannel symbol or a constellation subset identifier indicating a subsetof constellation points including the constellation point representingthe channel symbol,

wherein said demodulating and/or said decoding is configured to use saidredundancy data to demodulate the respective channel symbol and todecode the respective codeword, respectively.

According to a further aspect there is provided a broadcast systemcomprising:

a broadcast transmitter that transmits, via said broadcast system, areceiver input data stream comprising a plurality of channel symbolsrepresented by constellation points in a constellation diagram,

a receiver as disclosed herein that receives data transmitted by saidbroadcast transmitter,

a broadband server that provides redundancy data via a broadband systemfor reception by said receiver

According to still further aspects a computer program comprising programmeans for causing a computer to carry out the steps of the methoddisclosed herein, when said computer program is carried out on acomputer, as well as a non-transitory computer-readable recording mediumthat stores therein a computer program product, which, when executed bya processor, causes the method disclosed herein to be performed areprovided.

Preferred embodiments are defined in the dependent claims. It shall beunderstood that the claimed receiving method, the claimed broadcastsystem, the claimed computer program and the claimed computer-readablerecording medium have similar and/or identical preferred embodiments asthe claimed receiver and as defined in the dependent claims.

One of the aspects of the disclosure is to make use of the known conceptof using redundancy data (in the known system obtained via a unicastsystem, but generally obtainable via a broadband system) and to furtherdefine in which way redundancy data can best be obtained. It has beenfound that one or more least robust bits of the channel symbol or aconstellation subset identifier indicating a subset of constellationpoints including the constellation point representing the channel symbolare very efficient ways of providing redundancy data which are used toimprove the demodulation and/or decoding at the receiver.

It is to be understood that both the foregoing general description ofthe invention and the following detailed description are exemplary, butare not restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a schematic diagram of a broadcast system according to thepresent disclosure,

FIG. 2 shows a schematic diagram of a first embodiment of a receiveraccording to the present disclosure,

FIG. 3 shows a more detailed diagram of a broadcast system according tothe present disclosure,

FIG. 4A is a diagram illustrating the use of least significant bits asredundancy data according to the present disclosure,

FIG. 4B is another diagram illustrating the use of least significantbits as redundancy data according to the present disclosure,

FIG. 4C is a further diagram illustrating the use of least significantbits as redundancy data according to the present disclosure,

FIG. 5A is a diagram illustrating the use of puncturing according toaspects of the present disclosure,

FIG. 5B is another diagram illustrating the use of puncturing accordingto aspects of the present disclosure,

FIG. 5C is a further diagram illustrating the use of puncturingaccording to aspects of the present disclosure,

FIG. 6A is a diagram illustrating the use of a constellation subsetidentifier as redundancy data according to aspects of the presentdisclosure,

FIG. 6B is another diagram illustrating the use of a constellationsubset identifier as redundancy data according to aspects of the presentdisclosure,

FIG. 7 shows a diagram illustrating the identification of subcarriershaving a low CNR,

FIG. 8 shows a schematic diagram of a second embodiment of a receiveraccording to the present disclosure,

FIG. 9 shows a schematic diagram of a third embodiment of a receiveraccording to the present disclosure,

FIG. 10 shows a diagram of the relationship between overall cost fortransmission and transmitter power consumption,

FIG. 11 shows a schematic diagram of a dynamic broadcast system usingthe present invention,

FIG. 12 shows a schematic diagram of a control device for use in abroadcast system according to the present disclosure,

FIG. 13 shows a schematic diagram of another embodiment of a broadcastsystem according to the present disclosure,

FIG. 14 shows a schematic diagram of another embodiment of a broadcastsystem according to the present disclosure,

FIG. 15 shows a diagram illustrating the average estimated MutualInformation with and without knowledge of the transmitted bits,

FIG. 16 shows a schematic diagram of another embodiment of a broadcastserver according to the present disclosure,

FIG. 17 shows a diagram illustrating cooperative decoding with a servercontrolling the data exchange, and

FIG. 18 shows a diagram illustrating cooperative decoding without aserver controlling the data exchange.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, FIG. 1shows a schematic diagram of a broadcast system 1 according to thepresent disclosure. It comprises a broadcast transmitter 2 thattransmits, via said broadcast system, a receiver input data streamcomprising a plurality of channel symbols represented by constellationpoints in a constellation diagram. Further, it comprises one or morereceivers 3, in this case two receivers indicated as “user A” and “userB” arranged at different distances from the broadcast transmitter 2,according to the present disclosure for receiving data transmitted bythe broadcast transmitter 2. Still further, the broadcast system 1comprises a broadband server 4 (also called broadband provider), in thiscase a redundancy server that provides redundancy data via a broadbandsystem for reception by said receiver. Due to the use of transmission ofdata via broadcast and via broadband the broadcast system 1 may also becalled a hybrid broadcast system or a broadcast broadband system.

FIG. 2 shows a schematic diagram of a receiver 3 according to thepresent disclosure. It comprises a broadcast receiver 31 that receives,via said broadcast system 1, a receiver input data stream comprising aplurality of channel symbols represented by constellation points in aconstellation diagram. A demodulator 32 demodulates said channel symbolsinto codewords and a decoder 33 decodes said codewords into output datawords. A broadband receiver 34 obtains redundancy data via a broadbandsystem, said redundancy data for a channel symbol including one or moreleast robust bits of the channel symbol or a constellation subsetidentifier indicating a subset of constellation points including theconstellation point representing the channel symbol. According to thepresent disclosure said demodulator 32 and/or said decoder 33 isconfigured to use said redundancy data to demodulate the respectivechannel symbol and to decode the respective codeword, respectively.

In the proposed scheme the transmission in terrestrial network remainsunchanged, but for a poor reception the receiver (also called terminaldevice) can fetch additional data via the broadband network to improveerror correction performance. The receiver evaluates the data receivedfrom the terrestrial network, and according to the signal quality itrequires certain amount of additional data to assure quasi-error-free(QEF) reception. Under more severe conditions more additional data isneeded. In this way, for instance, a smooth or seamless transitionbetween pure terrestrial broadcast and complete delivery via thebroadband network can be realized. This creates a new degree of freedomfor the broadcast network management and may reduce the overall deliverycost and energy consumption.

The data received via both networks is combined for decoding in thereceiver. What kind of additional data is transmitted via the broadbandnetwork depends on the technology used in the terrestrial broadcastnetwork. FIG. 3 illustrates the proposed broadcast system 1 in moredetail, employing the proposed Redundancy on Demand (RoD) concept on theexample of DVB-T2. A RoD capable terminal (i.e. a receiver according tothe present disclosure) 3 a is equipped with a RoD client 34′, thatsubstantially corresponds to the broadband receiver 34 (see FIG. 2),that performs a request to the RoD server (i.e. the broadband server) 4if the reception conditions do not allow for error free decoding basedon the data received from the broadcast transmitter 2. The RoD server 4is then transmitting the required amount of redundancy, which isgenerally generated from the initially transmitted data stream, to thereceiver 3 a. Different convergence levels for generating the RoD dataare possible, i.e. the transmitted redundancy can either be generatedfrom the output of the multiplexer (MUX), the channel-coding or themodulation block. The proposed RoD scheme is backwards compatible, sincereceivers that are not capable of a broadband connection for improvingthe reception remain unchanged, such as the receiver 5 shown in FIG. 3.

A known approach for generating redundancy, which is described in WO2011/080020 A1, is the retransmission of erroneously received packetswith the so cold Automatic Repeat Request (ARQ) scheme. The generationand reinsertion of redundant packets therefore takes place in themultiplexer included in the modulator of the broadcast transmitter 2(see FIG. 3). Possible convergence levels are e.g. IP-Packets, FECFrames or Generic Stream Encapsulation (GSE) Packets for DVB-Systems.However, a drawback of this approach is the reduced granularity forgenerating the redundancy. If the reception conditions are slightlyworse than required for error free reception (e.g. 1 dB below the targetSNR), each packet needs to be retransmitted via the broadband systemrequiring a lot of transmission capacity.

Another approach as proposed according to the present disclosure is theusage of the least robust bits (or, generally, the bits having thehighest bit error rate, BER), in particular the least significant bits(LSB), of the used constellation (e.g. of a QAM constellation) asredundancy data. The receiver demodulates the QAM constellations, butuses the least robust bits, e.g. the LSBs, from the broadband networkinstead of the ones from the terrestrial broadcast network, because theleast robust bits typically carry the lowest amount of informationwithin the channel symbol (e.g. a QAM symbol).

As the bits from the broadband network are very reliable, thedemodulator 32 (in particular the demapper included within thedemodulator in various embodiments, e.g. used in OFDM receivers) is ableto reduce the number of possible constellation points. Thus, the averageEuclidean distance between the remaining constellation points increases,leading to improved performance. This approach is shown in an example inFIG. 4 according to which the LSBs are considered as least robust bits.The LSBs received via the broadband network in the constellation diagramshown in FIG. 4A is 0. The crosses 40 denote the possible constellationpoints and the lines 50 show the decision thresholds. In theconstellation diagrams shown in FIGS. 4B and 4C two LSBs are known (00and 01), reducing the remaining constellation points 41, 42 (indicatedby crosses) to four and reducing the number of decision thresholds 51,52. The same principle also holds for soft-decision demapping. In thiscase the knowledge of the redundancy data is used to enhance thereliability of the soft-decision output values of the demapper.

In another embodiment the demodulator 32 is configured to use said leastrobust bits of a channel symbol received as redundancy data to replacethe least robust bits of the channel symbol received by said broadcastreceiver to obtain an improved channel symbol and to demodulate saidimproved channel symbol.

In still another embodiment the receiver can also utilize the leastrobust bits (e.g. the LSBs) from the broadband network for improving thedemapping and/or decoding of the more robust bits (e.g. the MSBs) fromthe broadcast network. This concept is similar to “Genie-aided”demapping, i.e. an easily implementable soft-decision demappingapproach, where the knowledge of the transmitted bits is used to enhancethe reliability of the soft-decision output values of the demapper.Thus, the decoder 33 is configured according to this embodiment to usesaid least robust bits of a channel symbol received as redundancy datato replace obtained input values of the decoder with their ideal valuesderived from the known bits contained in the redundancy data and, thus,to obtain an improved codeword and to decode said improved codeword.Typically, the input values of the decoder are soft-decision inputvalues (generally log-likelihood ratios, LLRs) so that obtaineduncertain soft-decision input values are set to the ideal values havingindefinite likelihood (i.e. perfect knowledge) which are then used inthe decoding.

The bit rate of the generated redundancy data can be controlled firstlyby selecting the number of least robust bits out of each channel symbol,and secondly by puncturing the complete least robust bit stream, asillustrated in FIG. 5. Here, FIG. 5A shows a redundant data stream withbitrate R, FIG. 5B shows a redundant data stream with bitrate R/2, andFIG. 5C shows a redundant data stream with bitrate R/3. This isimportant to control the optimum amount of redundancy data. On the onehand the amount of transmitted redundancy data must be sufficient toallow for error-free decoding and/or demodulation, but on the other handthe amount should be as low as possible to avoid transmission ofunnecessary redundancy data.

Instead of retransmitting the initially transmitted bits, it is alsopossible to define constellation point subsets that are excluded in thereceiver. This allows for increasing the Euclidian distance between theremaining constellation points. If bit sequences with unequalprobabilities are transmitted, Huffman coding may be conducted on theselected bits, to enable the receiver to separate them easily and reducethe overhead for the transmission. This is meaningful if constellationshaping is used, altering the probabilities of the subset identifiers,i.e. some subset identifiers occur with higher probability than others.This is directly related to constellation shaping of (e.g. QAM)constellations, with some constellation points occurring with higherprobability than others. In the normal case of equal probabilities,Huffman coding does not provide any gain.

An example of a Huffman coding for a 16-QAM constellation is as follows:

Original bit sequence Probability Huffman coded bits XXX0 ½  0 XX10 ¼ 01X0X1 ¼ 10 1111 1/16 11

After the identification of the sub-carriers with heavy distortion,redundant bits will be generated from the constellation points of thesesub-carriers. Since the distortion levels of these identifiedsub-carriers may also vary, the redundant bits required for eachsub-carrier may also be different.

As an example, it shall be supposed that all the N constellation pointsbuild a set S. Through the re-labeling S will be divided into sub-sets{S1, S2, . . . , Sm}, i.e. not bits but (e.g. QAM) subset identifiersare transmitted. The division operation increases the average robustnessof the constellation points within each sub-set. This operation isdependent on the statistical properties of the broadcast signal and thedistortion. The mathematical deviation is omitted here. A simple exampleusing 16 constellation points of a QAM constellation is shown in FIG. 6.If the receiver receives a “0” (i.e. 16-QAM Gray Mapping with LSB knownto be 0), it indicates the original point locates in a first sub-setcomprising the constellation points 60 (indicated by crosses) as shownin FIG. 6A. If the the subsets are optimized for maximum averageEuclidian distance, the original point is located in a sub-setcomprising the constellation points 61 (indicated by crosses) as shownin FIG. 6B.

The various division schemes for different distortion levels and formscan be pre-calculated and stored in a look-up table to simplify theon-line operation. This approach enables a maximal utilization of thecapacity of the communication channel

In a multi-carrier communication system, the distortion each sub-carrierbears varies much in both time domain and frequency domain. In case ofportable mobile and stable receptions, the broadcast channel hasslow-changing and slow-fading characteristics. Furthermore, low signalpower and narrow band interferences are the main hinders for anerror-free reception. FIG. 7 illustrates such a broadcast channelexemplarily, in particular the variance of signal strength in frequencydomain. The first step is to identify the sub-carriers which are bearingsevere distortion and have hence an insufficiently lowcarrier-to-noise-ratio (CNR). In FIG. 7, they are the ones in the markedarea 70. Redundancy data is then only needed for these identifiedsub-carriers. In this way, the required bandwidth in the broadbandnetwork is reduced. Because the channel state may also changetemporally, the identification process is carried out periodically or inan event-based manner.

For the sub-carriers with low CNR, some or even all of the bits fromtheir constellation points need to be transmitted by the broadbandconnection, so that a correct decoding can be achieved. The selection ofthe bits as redundancy depends on the distortion, the strength of thesignal, the deployed mapping scheme. Moreover, which additional bitsshould be selected as redundancy data is also dependent on thepreviously selected bits. A mathematical derivation is omitted here.

An embodiment of a receiver 3 b according to the present disclosuremaking use of this approach in a more general way is shown in FIG. 8. Inaddition to the receiver 3 it comprises a quality detector 35 thatidentifies the quality of received channel symbols and a broadbandrequest unit 36 that requests channel symbols having the lowest qualityand/or a quality below a predetermined quality threshold as redundancydata.

In a further embodiment, as explained above with reference to FIG. 7,said broadcast receiver 31 is configured to receive said receiver inputdata stream via a multi-carrier broadcast system, e.g. an OFDM broadcastsystem (such as a broadcast system in accordance with a DVB standard).Said receiver input data stream comprises a plurality of channel symbolscarried by multiple frequency sub-carriers. In this embodiment thequality detector 35 is configured to identify the quality of saidfrequency sub-carriers, and said broadband request unit 36 is configuredto request channel symbols carried by sub-carriers having the lowestquality and/or a quality below a predetermined quality threshold asredundancy data.

In an embodiment the broadband receiver 31 is configured to receiveredundancy data via a broadband system. Thus, the broadband server orany other appropriate unit that is able to transmit data through thebroadband system thus actively transmits the redundancy data to thereceiver. For instance, it can be estimated, e.g. by the broadbandtransmitter or the broadcast transmitter, if the decoding and/ordemodulation at a receiver will be erroneous due to the channelcharacteristics, so that actively redundancy data will be sent, evenwithout explicit request by the receiver. Further, channel informationcan be used to select the redundancy data requiring the smallest amountof additional redundancy data.

In another embodiment, as shown in FIG. 9, the receiver 3 c comprises abroadband request unit 37 that requests, if demodulation of a channelsymbol and/or decoding of a codeword without redundancy data iserroneous, one or more least robust bits or constellation subsetidentifiers of the corresponding channel symbol via said broadbandsystem as redundancy data. Thus, the receiver 3 c actively requestsredundancy data in this embodiment.

Said broadband request unit 37 may alternatively, in another embodiment,transmit receiver specific broadcast channel information via saidbroadband system to a server (e.g. as shown in FIG. 3) that determinesthe quality of least robust bits and/or channel symbols received by thebroadcast receiver 31 and transmits least robust bits and/or channelsymbols as redundancy data to the receiver via said broadband system.Further, preferably, said broadband request unit 37 is configured tocompress said channel information by precoding before transmitting it toa server.

In an embodiment the channel state information (CSI) is estimated in thereceivers, but the identification of the sub-carriers and theconstellation points re-labelling can be carried out in either receiversor the server. If the receiver makes the decisions, it needs onlytransmit back a request for the specific bits. If the server makes thedecisions, the CSI should be transmitted back from the receiver to theserver. This CSI can be pre-coded before transmitting. For instance, theCSI can be transmitted incrementally, which means only the difference tolast estimation is required by the server. In this case, the CSI canalso be seen as two-dimensional spaces with the axis frequency and time.Furthermore, the CSI has specific characteristics in both dimensions,e.g. the variation in the time direction may be very slow in case ofstationary reception. Therefore, one could use algorithms similar toMPEG video encoding (e.g. differential encoding) that take benefit ofthese characteristics for an efficient transmission of the CSI back tothe transmitter.

In the following some application scenarios in which embodiments of thepresent disclosure can be applied will be explained.

When a poor QoS is noticed by the receiver and the availability of theredundancy data is proved, a question may be popped up to the userasking about whether acquiring redundancy data via broadband network isallowed to improve the decoding quality. Once the user accepts this ordoes choose for permanent allowance, the presentation of the currentmedia content is paused and the broadcast data stream is buffered for avery short interval to equalize the delays in both networks. Then thebroadcast data stream and redundancy data stream are synchronized anddecoded jointly. Afterwards the media content is displayed without anyperceptive errors.

Another advantage of the use of redundancy data is the possibility toefficiently counteract time varying distortions like man-made-noise.Man-made-noise is known to be especially severe in the VHF-Band, causingnarrowband as well as broadband distortions that can be either constantover time or time varying. Especially the impact on the QoS of timevarying distortions like impulsive noise, which are e.g. caused byswitching events of the mains, can be avoided by means of the concept ofredundancy data. Heavy noise impulses typically cause a service dropout,since the error correction (e.g. the forward error correction (FeC) asapplied in DVB broadcast systems) is not able to correct such strongdistortions. This is especially the case for OFDM-based systems, as ashort noise impulse in the time domain distorts a complete OFDM symbolin the frequency domain, due to the so called noise-buck effect of OFDM.The VHF band is therefore not used anymore in some countries (likeGermany) for digital terrestrial transmission, because of problems withman-made-noise. The use of redundancy data could allow for thereintroduction of terrestrial broadcasting in the VHF-band in suchcountries.

In current broadcast networks, to achieve a “quasi-error-free” (QEF)viewing experience, a certain level of signal-to-noise ratio (SNR) isrequired. For receivers which have a poor receiving condition, this SNRthreshold cannot be reached, thus a successful decoding of the broadcastsignal would be impossible. The TV services can only be providedcompletely by other means, e.g. IPTV and the corrupted broadcast signalhas to be discarded. For the proposed concept of using redundancy data,however, the amount of additional redundancy is dependent on the qualityof the broadcast signal: if the distortion is heavier, more redundancywill be needed; if lighter, then less redundancy will be needed. In aworst case, if the desired amount of redundancy data is even larger thanthat of the original TV content itself, the TV content will be delivereddirectly to the receiver as a normal IPTV system (100% broadband).Therefore, the data size that have to be transmitted as redundancy datais always less than without using this approach.

Compared to traditional systems, with the concept of redundancy data asoft-transition between pure broadcast and complete IPTV can berealized. This has an influential impact on the network planning, when ahigh proportion of the receivers are capable of using this concept. Forinstance, if a certain area shall be covered with a terrestrialbroadcasting network and a certain data rate shall be provided in thebroadcast channel Previously, after selecting the network setting (coderate, modulation scheme, etc.) the transmitter power had to be increasedto ensure a sufficient SNR at each location in this area. Now, with theconcept of redundancy data the transmitter power can be reduced and thereceivers located at edge area can be served with some redundancy datavia broadband network, so that their demodulation and decoding of thebroadcast signal is also “quasi-error-free”. Equivalently, the sametransmitter power can be maintained, but modulation and error correctionrates with higher spectral efficiencies could be applied. Thetransmitter power can be reduced to a level, when a further decrease ofpower consumption or transmission cost is no more possible. This powerlevel and the achievable power or cost savings are generally determinedby the viewer number, distribution of receivers, cost factors,man-made-noise and so on. Most of these parameters are changingtemporally, therefore online monitoring, optimization and adaption arepreferably used. For instance, the transmitter power is set to lowerlevel during morning, when less people are watching television. Thelower signal strength is compensated by more redundancy data via thebroadband network for the relatively small number of receivers.

The relation between the overall power consumption/transmission cost andthe transmitter power may look as depicted in FIG. 10. The optimaloperation point moves at different time and changes for differentnetwork settings, such that a dynamic adaption of the network ismeaningful.

The proposed ideas can also be applied in a dynamic broadcast system as,for instance, described in U.S. patent application Ser. No. 13/428,743filed on Mar. 23, 2012, which description is herein incorporated byreference. The so-called dynamic broadcast concept applied in such adynamic broadcast system describes a flexible terrestrial broadcastsystem, utilizing an internet broadband connection and a hard disc inthe terminal, to optimize the spectrum usage and the power consumptionof the transmitter network, by choosing the optimal delivery meansdepending on the content type (realtime, non-realtime) and the viewercount. The relationship of the concept of redundancy (on demand) withrespect to the dynamic broadcast concept is depicted in FIG. 11 showinga schematic diagram of a dynamic broadcast system 100 according to thepresent disclosure.

The system 100 involves a broadcast (BC) network, a broadband network(BB), hybrid broadcast broadband (HBB) terminals (receivers) and otherwireless communication networks. Their cooperation is managed by thedynamic broadcast network. The functions of the blocks shown in FIG. 11are explained in the following.

First, the packaging of media content unit 112 is described. The TVcontent is provided by broadcasters 110 and is segmented into Real-Time(RT) and Non-Real-Time (NRT) events. For real-time events, (certainelements of) news programs for instance, their content becomes availableonly at the announced on-air time, so they have to be delivered live;while for non-real-time events, like movies, music, drama etc., theircontent may be available in advance, so they can be pre-downloaded. Withpre-download (broadcast or broadband) network capacity can be used forinstance over night, when capacity has been identified to be available,whereas during daytime and in the evening network capacity will be freedfor other uses. The choice of content that can be pre-downloaded will bebased on rules used in a decision logic 114. These rules will begenerated from usage patterns of viewers derived from informationavailable over the broadband network. In conjunction with other measuresthe download of such material will take place as network capacitybecomes available—either over the broadcast or the broadband network. Aprogram schedule therefore should be created that indicates whichcontent comes over the air in real-time and which content can be playedfrom the storage device in the user terminal.

Next, a monitoring and signaling unit 116 is described. To optimize thenetwork operation, knowledge about actual network usage is important.Two kinds of information should hence be collected from HBB terminals118 (also called “terminals”, “user terminals” or “receivers”hereinafter) and transmitted to the decision logic 114 through broadbandconnection. The first kind of information is about whether or notprograms or pieces of media content are used and by how many people.This popularity can be estimated by monitoring the watching activitiesof some or all users, as done in today's IPTV networks. Knowing theaccurate popularity and usage pattern of the media content can help thedecision logic 114 determining which content should be delivered via thebroadband network and/or pre-downloaded as mentioned above. The secondkind of information is about the momentary technical Quality of Service(QoS) of the transmission links. This can be obtained with integratedmeasuring devices in HBB terminals 18. With information about the actualsignal quality, the decision logic 14 can manage the network mostefficiently.

The signaling which delivers data to the HBB terminals 118 will provideinformation about content items presented for ‘delivery in advance’(also called ‘offline delivery, i.e. delivery in advance of the officialbroadcast time), the time of the broadcast transmission and/or the timeof play out over the broadband network. It will include a programschedule and it will deliver information about the various parametersselected by the dynamic multiplexing and a joint control unit 120. Thesignaling information can be transmitted via both networks and in bothpush and pull modes, so that the HBB terminals 114 can get the currentnetwork information even if it is just switched on for the first time.

The decision logic 114 is in charge of the management of the wholenetwork and it aims to keep the operation at a minimal cost whileassuring required QoS. Facilitated with the monitoring reports from theHBB terminals 118, and based on additional business rules, costfunctions, realistic constraints etc. the decision logic 114 may changethe packaging of real-time and non-real-time events, or command are-multiplexing of the transport streams in broadcast and broadbandchannels or adjust of the transmission parameters and transmitter power.Before the decision logic 114 has made any changes to the previousprogram schedule or network settings, it should acknowledge all HBBterminals 18 about the modification through signalling.

Next, a multiplexing and content distribution unit 122 is described. Theflexible distribution of media content through broadcast and broadbandnetwork requires content items and complete or partial audio, data andvideo programs to be multiplexed dynamically. In consequence, the formerfixed mapping between transmission parameters and TV programs has to beeliminated. Information about such re-multiplexing should be signaled tothe HBB terminals 118, so that they are able to follow the changes. Bythe reason that the popularity of the different TV programs in onetransport stream changes continuously, re-multiplexing may take placeonline, which means some content being transmitted may be reallocated inother physical channels or still in the current channel but with newtransmission parameters. All these actions should be carried out in away unnoticeable by the users.

Next, the joint control unit 120 for control of transmission parametersis described. In traditional digital broadcast systems the modulation ofthe transmitted signal and the degree of Forward Error Correction (FEC)used are decided once and they then stay stable. The transmitter poweris selected according to the coverage requirements of the network. Interrestrial networks, the coverage area is defined by the aforementionedparameters and in addition by the coverage pattern determined by thetransmit antenna. This static network planning leads to inefficientusage of the valuable spectrum, because strong time-variant factors likechannel popularity and user terminals receiving conditions have not beentaken into consideration.

Dynamic multiplexing can reduce the useful data rate transmitted on aspecific channel if the multiplex on that channel is not fully loadedwith program items at the moment. Initiated by the decision logic 114the joint control unit 120 will then change the FEC settings and/ormodify the modulation scheme used on that channel. This will result inan enhanced robustness of the signal which in consequence will allow thetransmitter power to be adapted thus reducing the power density—and thecost of transmission. This creates economical benefits, as well asecological benefits, since the exposure to radiation and carbon emissionwill be reduced as a consequence of the lowered transmitter power. Inanother case, it shall be supposed that signaling provided from userterminals to the broadcast network including information about technicalparameters of the received signal in networks indicate abetter-than-required or worse-than-required signal quality as a resultof changes in man-made noise (i.e. noise generated by any devices usedby anybody in the environment)—which has been found to fluctuate greatlyand periodically over time—or due to changes in weather conditions.Initiated by the decision logic 114 the joint control unit 120 willmodify the parameters (FEC, modulation, transmitter power) in order toaccommodate broadcast QoS at a minimum cost. In addition, the jointcontrol unit 120—in negotiation with dynamic multiplexing via thedecision logic 114—will initiate the re-configuration of multiplexessuch that the data rate transmitted in heavily disturbed channels willbe reduced and the robustness of the signal enhanced as required.

In the HBB terminal 118 some content will have to be stored “off-line”upon receipt of the appropriate downstream signaling and besides, whichcontent to store should also be decided by the HBB terminal 118.Therefore it should be capable of predicting user's preferences, storingrelevant TV content automatically and managing the stored contentdynamically. To accomplish this, a recommender system should beimplemented in the HBB terminal 118. On the other hand some content willbe made available via the co-operating broadband network. The HBBterminal 118 will receive a program schedule, and a delivery networkindicator which indicate for which period of time and how often thisstored content is to be used instead of content that in traditionalbroadcasting would be received live. In addition it will be informed viawhich of the co-operating networks content will be delivered. Thereceived content from different networks should be managed properly bythe HBB terminal 118. Content items are often interrelated. This isobviously true for audio and video but in addition, a plethora of dataservices like software applications are created by the content ownersthat will have to be available in the terminal 118 and started, pausedor cancelled in relation to the audio and video content. Additionaldownstream signaling information embedded in the broadcast stream isreceived by the HBB terminal 18, which indicates the dynamic multiplexconfigurations and the parameters selected by joint control. Upstreamsignaling will be generated in HBB terminals 118 for transmission on thebroadband network. The user terminal 118 thus becomes an activecomponent of the dynamic broadcast network instead of being a passivedevice as in traditional broadcasting.

Spectrum freed by dynamic broadcast can be offered to secondary wirelessnetworks, like Cellular (LTE), Wi-Fi, etc. for a certain period of time.To avoid interference, usage of the new “white space” created by dynamicbroadcast should be coordinated through resource signaling which is anoutput of the dynamic broadcast system 100 and informs wireless networkoperators about the dynamically chosen parameters of the broadcastnetwork. It includes also information about the period of validity ofthe multiplex configuration and the spectrum resources which will befreed including an indication of the period of time during which thespectrum will be available.

More details about the general concept of dynamic broadcast can be foundin the above mentioned US patent application and other publicationsabout dynamic broadcast systems.

As the concept of redundancy on demand provides a “seamless transition”option between complete broadcast or broadband transmission, it can beefficiently combined with the dynamic broadcast concept, introducinganother degree of freedom, so that the dynamic broadcast network can befurther optimized in the sense of transmission cost, energy consumption,and spectrum efficiency. This is indicated in FIG. 11 by the arrowoutput by the joint control unit 120 that controls the output of RoDdata via the broadband network to the HBB terminal 118.

Redundancy data can also be used in another application as an encryptionway to protect the pre-downloaded media content. The pre-download of themedia content can be transmitted with a network configuration of highdata rate but week error correction. The redundancy data can then beused as a triggering signal to enable the recovery of the original data.

Further, conditional access to data can be realized by use of redundancydata. Conditional access to video services is crucial for pay-TVtransmission. Redundancy data can be used to control the access topay-TV services by means of the broadband connection. The correspondingservice is transmitted via terrestrial broadcast to achieve low networkcost. However, not the full data rate is transmitted via terrestrialbroadcast, but only a specific amount, e.g. like 95% of the data rate.The users that subscribed to the pay-TV service then receive theremaining 5% via the broadband connection as redundancy data. Thisallows the network operator for restricting the access to the pay-TVservice via the broadband connection only to the users with thecorresponding service subscription. Other users without the additionalredundancy data over broadband are not able to decode the service, sincethe received Mutual Information via terrestrial broadcast is notsufficient for error free decoding. For this purpose even a slightdecrease of the transmitted Mutual Information over the terrestrialbroadcast suffices to avoid access of unregistered users to the pay-TVservice.

A schematic diagram of a control device 200 for use in a broadcastsystem according to the present invention is shown in FIG. 12. Such acontrol device 200 may e.g. be used as joint control unit 120 in thebroadcast system 100 shown in FIG. 11. The control device 200 comprisesa broadcast control unit 201 and a broadband control unit 202. Thebroadcast control unit 201 controls a broadcast transmitter of saidbroadcast system that broadcasts broadcast signals in a coverage areafor reception by terminals comprising a broadcast receiver and abroadband receiver. The broadband control unit 202 controls a broadbandserver of a broadband system that provides redundancy data to terminalswithin said coverage area. The broadband control unit 202 is configuredto control the provision of redundancy data by said broadband server foruse by one or more terminals which use said redundancy data togetherwith broadcast signals received via said broadcast system for recoveringcontent received within said broadcast signals and/or provided via saidbroadband system. Additional optional elements are shown in dashed boxesand will be explained below.

In an embodiment said broadcast control unit 201 is configured to changeone or more transmission parameters of said broadcast transmitterdepending on one or more parameters of a group of parameters comprisingthe time of the day, the number, location, profile and/or parameters ofactive terminals, cost factors of the transmission of data by saidbroadcast transmitter and/or said broadband server, channel stateinformation (in particular noise and/or reception level) and/or feedbackof terminals. Further, said broadband control unit 202 is configured toprovide redundancy data to one or more active terminals which receivebroadcast signals with insufficient quality and which use saidredundancy data to compensate for the insufficient quality of receptionof broadcast signals. Optionally, the control device 200 furthercomprises a monitoring unit 203 that continuously or repeatedly monitorsone or more parameters of said group of parameters.

In another embodiment said broadcast control unit 201 is configured tocontrol the transmit power and/or one or more physical layer parameters,in particular modulation and/or code rate, parameters of an interleaverand/or an FFT unit, used by said broadcast transmitter.

In another embodiment said broadcast control unit 201 is configured toadaptively change the transmit power and/or the efficiency of theapplied modulation and/or code depending on one or more parameters of agroup of parameters comprising the time of the day, the number,location, profile and/or parameters of active terminals, cost factors ofthe transmission of data by said broadcast transmitter and/or saidbroadband server, channel state information (in particular noise and/orreception level) and/or feedback of terminals.

In another embodiment said broadband control unit 201 is configured toadaptively change the amount of redundancy data transmitted to one ormore active terminals depending on one or more parameters of a group ofparameters comprising the time of the day, the number, location, profileand/or parameters of active terminals, cost factors of the transmissionof data by said broadcast transmitter and/or said broadband server,channel state information, noise and/or feedback of terminals,preferably depending on the number of active terminals. Preferably, inthis embodiment the broadcast control unit 201 is configured to reducethe transmit power and/or to apply a modulation and/or code with ahigher efficiency and said broadband control unit is configured toincrease the amount of redundancy data transmitted to one or more activeterminals if the number of active terminals is below a lowerpredetermined threshold and/or to decrease the amount of redundancy datatransmitted to one or more active terminals if the number of activeterminals is above an upper predetermined threshold. Even further, theamount of redundancy data may be controlled based on the costs of thetransmission, i.e. based on an estimation if it is more cost efficientto increase or decrease the amount of redundancy data versus use ofbroadcast for transmitting data.

Still further, in an embodiment the control device 200 comprises anoptional request receiving unit 204, as also shown in FIG. 12, thatreceives requests for transmission of redundancy data from terminals. Inthis embodiment said broadband control unit 202 is configured to controlthe broadband server to provide redundancy data to requesting terminals.The requests from terminals may generally differ in the quantity ofrequested redundancy data, the quality of the requests, the useprofiles, etc. For instance, there may be premium users (which may havepaid an extra service charge), which may always receive an extra amountof redundancy data in order to ensure a high quality of the transmissionin all situations.

In another embodiment the control device 200 is particularly designedfor use in a dynamic broadcast system as shown in FIG. 11, wherein saidbroadcast control unit 201 and said broadband control unit 202 areconfigured to dynamically control transmission parameters, transmissiontimes and transmission paths used for broadcasting and providing contentby use of said broadcast transmitter configured to broadcast content viasaid broadcast system and/or said broadband server configured to providecontent via said broadband system. In this embodiment the control device200 further comprises an optional decision unit 205 (also shown in FIG.12) that dynamically decides transmission parameters, transmission timesand transmission paths used for broadcasting and providing content byuse of said broadcast transmitter and for providing content by saidbroadband server.

Said decision unit 205 is preferably configured to dynamically decidetransmission parameters, transmission times and transmission paths usedfor broadcasting and providing content based on monitoring data carryinginformation on user specific content usage and/or transmission qualitydata carrying information on the quality of a transmission link betweensaid broadband server and a terminal and/or of a reception of contentbroadcast by said broadcast transmitter.

Further, said redundancy data is preferably provided for providing aseamless transition between broadcast and broadband reception and/orrecovery of content from signals received via said broadcast system andsaid broadband system.

In another embodiment said broadcast control unit 201 and/or saidbroadband control unit 202 is configured to control said broadcasttransmitter and/or said broadband server to transmit content in a formthat does not allow complete recovery in a terminal without the use ofredundancy data, and to control the transmission of redundancy data viasaid broadband system to terminals that shall be enabled to completelyrecover received content.

Preferably, in said embodiment said broadcast control unit 201 and/orsaid broadband control unit 202 is configured to control said broadcasttransmitter and/or said broadband server to transmit content inencrypted form and/or with insufficient and/or low quality and whereinsaid redundancy data is provided for being used for decryption and/orincreasing the quality of the received content. For instance, in anexemplary use scenario, via broadcast a “normal” (lower) image quality(e.g. in SD format) is obtained, while by use of the redundancy data(which may then be regarded as “additional data” or “auxiliary data”)received via broadband an “improved” (higher) image quality (e.g. in HDformat) is obtained.

Further, in said embodiment said broadcast control unit 201 ispreferably configured to control said broadcast transmitter toadaptively change the mutual information between transmitted andreceived signals to transmit content in a form that does not allowcomplete recovery in a terminal without the use of redundancy data.

A broadcast system 1 a comprising such a control device 200 isschematically depicted in FIG. 13. The broadcast system 1 a comprises abroadcast transmitter 2 a that broadcasts broadcast signals in acoverage area for reception by terminals 3 comprising a broadcastreceiver and a broadband receiver. The broadcast system 1 a furthercomprises a broadband server 4 a that provides redundancy data toterminals within said coverage area. Finally, the broadcast system 1 acomprises a control device 200 as explained above with reference to FIG.12 that controls said broadcast transmitter 2 a and said broadbandserver 3 a.

In the following it will be described in more detail how the requiredamount of redundancy data can be estimated or determined. In particular,the estimation of the Mutual Information, the estimation of the numberof redundancy data bits and the stream synchronization will be describedby use of various embodiments. The following description will refer toelements shown in FIG. 14 depicting the interaction of a broadbandserver (called RoD server) 4 b and a receiver (called terminal in thisembodiment) 3 d used in a broadcast system 1 b according to the presentdisclosure.

Generally, a receiver (see also the embodiment of a receiver shown inFIG. 9) for receiving data in such a broadcast system comprises abroadcast receiver (31 in FIG. 9) that receives via said broadcastsystem a receiver input data stream comprising a plurality of channelsymbols represented by constellation points in a constellation diagram,a demodulator (32 in FIG. 9) that demodulates said channel symbols intocodewords and a decoder (33 in FIG. 9) that decodes said codewords intooutput data words. A redundancy calculator (not separately shown in FIG.9; may be a separate element or included in the broadband request unit37; separately provided as unit 38 in the receiver 3 d shown in FIG. 14)determines a required amount of redundancy data for correct demodulationand decoding by use of the originally received channel symbol andadditional redundancy data. A broadband request unit (37 in FIG. 9)requests, if demodulation of a channel symbol and/or decoding of acodeword is erroneous or likely to fail, a required amount of redundancydata via a broadband system and a broadband receiver (34 in FIG. 9)receives redundancy data via said broadband system. The demodulatorand/or the decoder are configured to use said redundancy data todemodulate the respective channel symbol and to decode the respectivecodeword, respectively. These elements are generally also provided inthe receiver 3 d shown in FIG. 14 even if not explicitly depicted.

The broadband server 4 b for providing redundancy data to a receiver ofsuch a broadcast system via said broadband system generally comprises areceiving unit 41 that receives requests from receivers of saidbroadcast system via said broadband system to provide redundancy data tothe respective receivers via said broadband system to enable correctdemodulation of a channel symbol and/or decoding of a codeword, arequest including channel state information, a redundancy calculator 42that determines the required amount of redundancy data required forcorrect demodulation and decoding by use of said channel stateinformation, and a transmitting unit 43 that provides redundancy data inat least said required amount to the receiver that requested redundancydata.

An essential task of a system using redundancy data is to correctlydetermine the required amount of redundancy data for successful decodingin the terminal (=receiver). If too few redundancy data is transferredfrom the redundancy provider (i.e. a broadband server) to the terminal,the decoding process will fail and additional redundancy data need to berequested in a second step. This causes network overhead and increasesthe system delay until successful decoding is achieved due to themultiple redundancy data requests. If on the other hand too muchredundancy data is transferred to the terminal, the system efficiency isreduced, since data is transmitted via the broadband connection in vain.The calculation of the correct redundancy data amount is therefore veryimportant, since it influences the performance of the overall system.

A possible metric for the estimation of the required redundancy dataamount in the receiver is the Mutual Information (MI) betweentransmitted (code) bits and received soft values, belonging to onecodeword (e.g. a FEC word). The Mutual Information is a figure of meritfrom stochastic and is especially suited for determining the requiredamount of redundancy data, since it is independent from the channelcharacteristics and the modulation order of the QAM constellation, butonly depends on the applied code. If the code rate of the applied codeis e.g. 0.5, decoding is successful if the Mutual Information exceedsthe value of 0.5. However, this only holds for an ideal encoder,operating at the maximum channel capacity (Shannon capacity), which isnot possible with practical error correction codes. For instance, theDVB-T2 64K LDPC code with a code rate 0.5 requires a Mutual Informationof 0.55 for successful decoding. There are only very slight deviationsin the performance of this code depending on the modulation order andthe channel characteristics. The required Mutual Information for theutilized codes can be stored in a table in the broadband server or theterminal, such that the required mutual information that needs to betransmitted via redundancy data can be calculated in the terminal or thebroadband server. Hence, in an embodiment the redundancy calculator 38is configured to estimate said required amount of redundancy data basedon channel state information and/or Mutual Information betweentransmitted and received data, in particular between transmitted bits ofoutput data words or codewords and received values representing bits ofoutput data words or codewords.

There are two locations in the receiver where the log-likelihood ratios(LLRs) can be extracted to calculate the Mutual Information: Eitherdirectly after QAM demapping or after FEC decoding. If the LLRs afterFEC decoding are used, less redundancy data needs to be transmitted inprincipal (because FEC decoding, though not successful, increases thereliability of the LLRs). Using the estimated Mutual Information it ispossible to estimate the meaningfulness to perform FEC decoding. Whenthe Mutual Information is clearly lower than the required MutualInformation for FEC decoding, FEC decoding should be omitted. This isthe case because on the one hand the Mutual Information increase by theFEC decoder is typically negligible in such situations, especially forstate-of-the-art FEC codes like LDPC or turbo codes, and on the otherhand this allows a reduction of the power consumption of the terminal.

The Mutual Information is determined based on the Log-Likelihood-Ratios(LLR) at the output of the QAM-demapper and is a good measure if thefollowing FEC is able to successfully decode the FEC codeword. An LLR isdefined here as

${inputLLR} = {\ln\frac{P\left( {{bit} = 1} \right)}{P\left( {{bit} = 0} \right)}}$The Mutual Information of a single Bit based on its LLR value is definedasIf transmitted bit=1: MI=1−log 2(1+e ^(−inputLLR))If transmitted bit=0: MI=1−log 2(1+e ^(+inputLLR)).

The Mutual Information is typically averaged over one FEC block todecide if successful decoding is possible. However, the knowledge of thetransmitted bit is required for the calculation, which is not availablein a receiver. To avoid the need for the reference data for thecalculation of the Mutual Information, the formula is weighted by thelinear probability that a 1 or a 0 is transmitted, respectively. Thelinear probability (in the range [0,1]) that a 1 is transmitted iscalculated from its LLR value by

$p = {\frac{1}{1 + e^{- {inputLLR}}}.}$

After weighting the initial Mutual Information formulas (assuming bit 1or bit 0 was transmitted) with the probability p and 1-p, respectively,the following formulas are resulting:MI₁=1−p*log 2(1+e ^(−inputLLR))MI₀=1−(1−p)*log 2(1+e ^(+inputLLR))The estimated Mutual Information without reference is then resultingfrom their sumMI_(estimated)=MI₁+MI₀=1−p*log 2(1+e ^(−inputLLR))+1−(1−p)*log 2(1+e^(+inputLLR))

The comparison of the Mutual Information estimation with its idealvalues is shown in FIG. 15 for different channel models and modulationsizes with a large amount of averaged bits and ideal channel knowledge.It can be observed that estimated Mutual Information exactly correspondsto the ideal Mutual Information. In practice, the Mutual Information isestimated for a particular codeword (or a Time Interleaver Frame,consisting of several codewords), which results in a smaller amount ofbits available for averaging. This would result in some degradation ofthe estimation. Other metrics to compute the amount of requiredredundancy can be the estimated signal-to-noise ratio (SNR), the averageabsolute value of the LLRs or the estimated modulation error rate (givenby the deviation of the received QAM symbols to the possible transmitQAM symbols).

Based on the estimated mutual information the required number of bitsfor the redundancy data transmission to the receiver needs to becalculated. This can be done without knowledge of the channel stateinformation (CSI), or taking the CSI into account. If CSI is availableat the broadband server, the bits that experienced strong attenuationfrom the transmission channel are preferably transmitted first. If noCSI is available this is not possible.

To allow for optimum performance of iterative FEC codes, the transmittedredundancy data bits should be uniformly distributed over the FECcodeword. This avoids that the transmitted redundancy data is onlylocated e.g. at the beginning of the FEC codeword. This can be achievedby means of a pseudo random address generator that generates theaddresses of the bits within the FEC codeword selected for transmission.Thanks to the random nature of the generated addresses the selected bitsare uniformly distributed within the FEC codeword. The random addressgenerator must be known to both the broadband server and the receiver,to allow for unambiguous decoding in the receiver based on thetransmitted redundancy data bits. In case of the transmission of leastrobust bits (e.g. LSBs) first, as explained in an embodiment above, therandom addresses of the least robust bits of all QAM symbols that carrya FEC codeword are used for generating the redundancy data bits first.Afterwards the second least robust bits are used, and so on, until therequired amount of redundancy data bits is reached.

The calculation of the amount of required redundancy data bits iscarried out in the receiver, based on the estimated Mutual Informationand the required Mutual Information for successful decoding of theutilized FEC code. The required Mutual Information is known for all coderates (see e.g. FIG. 15 for 64K LDPC of rate 1/2) by simulation and arestored in the server and the receiver. Depending on the resulting SNR ofeach received QAM symbol (determined by the CSI), the additional MutualInformation can be calculated that results in the receiver when aparticular bit is perfectly known. This additional Mutual Information isadded to the available Mutual Information for each pseudo randomlygenerated bit location until the threshold of the overall MutualInformation for error free decoding is reached. By this means, thenumber of required redundancy data bits can be assessed in the receiverand a request with this number of bits is sent to the broadband server.The broadband server then uses the same pseudo random address generatorto generate the redundancy data bits in the receiver.

As random address generator a linear feedback shift register (LFSR) witha polynomial representing a maximum length sequence (MLS) can be used.For instance for a FEC block size of 64800 the register values generatedby a 16-bit LFSR with a cycle length of 2¹⁶−1=65535 could be used.However, only register values smaller or equal to 64800 are used as bitaddresses, since the usage of the modulo-operator to truncate largervalues could lead to a manifold generation of the same bit address.Other algorithms like the Mersenne-Twister can be used as well, but arenot that simple to implement compared to an LFSR. Preferably, therequested bits are only information bits in case the FEC code issystematic. It shall be assumed that the channel completely erased acodeword (of N bits—where K bits are information bits, i.e., the coderate is K/N). In this case, instead of requesting the complete codewordagain (N>K), it would be sufficient to retransmit only the informationbits (K).

The required number of bits in the receiver can then be computed basedon the knowledge of the current Mutual Information. Iteratively newpseudo random bit addresses are generated of bits that are transmittedas redundancy. After each newly generated bit the additional MutualInformation is calculated that results from ideally knowing theadditional bit at the generated address in the receiver. The additionalMutual Information is easily accessible from a look up table that can bepre-computed by means of Monte Carlo Simulation. Based on the additionalMutual Information the current Mutual Information is updated by addingthe additional Mutual Information. This is iteratively repeated untilthe current Mutual Information exceeds the required Mutual Informationfor successful decoding. In pseudo code the algorithm for thecalculation of the number of required bits in the receiver is thefollowing:

  RoD bits to request = 0 while (current mutual information < requiredmutual information) {  generate bit address within FEC codeword;  lookup additional mutual information for QAM symbol from LUT  current mutualinformation = current mutual information +  additional   mutualinformation  RoD bits to request = RoD bits to request + 1; }

In short: The algorithm describes a method to estimate the number ofretransmitted pseudo random information bits (i.e. without utilizing thechannel state information) for error free decoding

All Mutual Information here corresponds to bitwise mutual information,such that the values are normalized to 1 to allow for direct comparisonwith the required mutual information value independent of the modulationorder.

The table of the additional mutual information per QAM symbol dependingon the number of known redundancy data bits within the QAM symbol isstored in the receiver as a look up table (LUT), e.g. in the storage 40.The additional mutual information depends on the SNR of the QAM symbolthat carries the bit, and the bits that are known within the QAM symbol.For instance a LUT that stores the additional mutual information for theSNR range of 1 up to 30 dB for 256-QAM requires 30*256=7680 entries. Ifit is assumed that the LSBs are transmitted first, and so on, only30*8=240 entries are required, since only 8 states are possible for eachQAM symbol (1 bit known, . . . , 8 bits known). The values of the LUTentries are determined by Monte-Carlo simulation in advance, based onthe formula of the ideal Mutual Information.

Since the LSBs of QAM symbols carry less Mutual Information and aretherefore well suited as redundancy data bits, it is meaningful tooptimize the algorithm such that it first generates the bit addresses ofthe LSBs, then the addresses of the bits with the next lower orderwithin the QAM symbols (LSB-1), and so on. By this means the bits withthe highest order, providing the most additional Mutual Information, aretransmitted first, reducing the required number of redundancy bits.

Thus, in such an embodiment the redundancy calculator 38 is configuredto estimate said required amount of redundancy data based on acomparison between estimated Mutual Information and required MutualInformation for correct demodulation and decoding. For estimating theMutual Information a mutual information estimation unit 39 is preferablyprovided. Further, in an embodiment a storage 40 is additionallyprovided that stores the required Mutual Information for a plurality ofcodes, in particular for a plurality of code rates and/or codewordlengths.

Accordingly the redundancy calculator 42 of the broadcast server 4 b ispreferably configured to estimate said required amount of redundancydata based on channel state information and/or Mutual Informationbetween transmitted and received data, in particular between transmittedbits of output data words or codewords and received values representingbits of output data words or codewords. Further, the redundancycalculator 42 is preferably configured to estimate said required amountof redundancy data based on a comparison between estimated MutualInformation and required Mutual Information for correct demodulation anddecoding. Still further, preferably a storage 44 is provided that storesthe required Mutual Information for a plurality of codes, in particularfor a plurality of code rates and/or codeword lengths.

The algorithm above requires a lot of computations to determine thenumber of required bits, since the mutual information must be calculatedfor every additional bit per QAM symbol, to reflect the actual noise ofeach QAM symbol. However, the algorithm can be simplified by assuming anaverage noise level throughout the FEC codeword. Based on the averagenoise level the average additional Mutual Information is calculated forthe current bit order (LSBs transmitted first). Based on the averageMutual Information the number of additional bits is calculated toprovide the amount of required Mutual Information for error freedecoding. If the number of required bits of this bit order is notsufficient, all bits of this bit order are flagged for transmission andthis is the same is then iteratively calculated for the next bit order,and so on. If the current bit order provides enough bits to bridge theremaining gap to the required Mutual Information, the number ofredundancy bits is calculated by adding the (N/M) bits for each bitorder that is completely transmitted, plus the additionally requiredbits of the current bit order. This algorithm requires only onecalculation for each of the M bit levels, instead of one calculation perbit, since the additional Mutual Information for each bit order isassumed to be the same throughout all (N/M) bits of this bit order. Thepseudocode for this simplified calculation of the required number ofbits is the following:

If (current MI < required MI) {  missing MI = required MI − current MI; for (int i = 0; i < M; i++) {   get additional MI from LUT (additionalMI per QAM symbol if i + 1 Bits are known    instead of only i Bits.assuming an average SNR for all QAM symbols)  calculate the number ofrequired QAM symbols to bridge the gap of missing MI if   one additionalbit is known  if (number of required QAM symbols < N/M) {   RoD bits torequest = i * (N/M) + number of required QAM symbols;    break;   }  else {   current MI = current MI + additional MI * (N/M)   missing MI= required MI − current MI;   }  } }

In short: The algorithm describes a method to estimate the number ofretransmitted pseudo random information bits (i.e. without utilizing thechannel state information) for error free decoding, with reducedcomputational complexity but also reduced accuracy compared to algorithm1.

Thus, in such an embodiment the redundancy calculator 38 of the receiver3 d is configured to determine an average additional Mutual Informationfor a channel symbol and to add said average additional MutualInformation to available Mutual Information for each bit location of aparticular codeword until a threshold of an overall Mutual Informationrequired for correct decoding is reached and to determine the requiredamount of redundancy data required for correct decoding of saidparticular codeword based thereon. Accordingly, the redundancycalculator 42 of the broadband server 4 b is configured to determine anaverage additional Mutual Information for a channel symbol and to addsaid average additional Mutual Information to available MutualInformation for each bit location of a particular codeword until athreshold of an overall Mutual Information required for correct decodingis reached and to determine the required amount of redundancy datarequired for correct decoding of said particular codeword based thereon.

Alternatively, the following calculation of the required number ofredundancy based on estimated Mutual Information can be used, if it isassumed that the redundancy consists of already transmitted code bits.

The estimated Mutual Information shall be denoted as MI_(old), therequired number of redundancy data as n, the number of bits in thecodeword as N (e.g., N=64800 in a 64 k LDPC). The new Mutual InformationMI_(new) after n (already transmitted) redundant bits have beenre-transmitted via a unicast system (generally, the broadband system) isthen obtained by:

${MI}_{new} = {{{\frac{n}{N} \cdot 1} + {\left( {1 - \frac{n}{N}} \right) \cdot {MI}_{old}}} = {\frac{n}{N} + {\frac{N - n}{N} \cdot {MI}_{old}}}}$

The n redundant bits from the broadband server are received withsubstantially perfect knowledge, since a unicast system can guaranteeerror-free transmission. The formula is due to the mixing property ofthe EXIT chart, see “A. Ashikhmin, G. Kramer, and S. ten Brink,“Extrinsic information transfer functions: model and erasure channelproperties,” IEEE Trans. Inform. Theory, vol. 50, no. 11, pp. 2657-2673,November 2004.

From the previous formula, it is obtained:

$n = \left\{ {{\begin{matrix}{N \cdot \frac{\left( {{MI}_{new} - {MI}_{old}} \right)}{1 - {MI}_{old}}} & {if} \\0 & {else}\end{matrix}\mspace{14mu}{MI}_{new}} > {MI}_{old}} \right.$n will be lower bounded by 0 (if MIold>MInew) and upper bounded by thenumber of information bits K in the codeword, if MInew is set to thecode rate (or slightly above) R=K/N, in case MIold=0.

In short: The formula computes the amount n of redundancy that has to beretransmitted. The new Mutual Information is computed, which is theweighted sum of the old Mutual Information and perfect MutualInformation for those n bits, and compared with the desired MutualInformation that is required for successful decoding.

If the CSI of the receiver is available at the broadband server, thecalculation of the required redundancy data bits can alternatively becarried out in the server. The receiver then first transmits the CSI tothe server (a possible CSI compression scheme is described below). Basedon the SNR of each QAM symbol (determined from the CSI), the server isable to find the bit in the FEC codeword by means of the LUT thatprovides the largest additional mutual information. This way the bitsthat experienced deep fading are used first as redundancy data bits,since the additional mutual information of these bits is the largest.The algorithm is very similar to the algorithm without CSI knowledge.The important difference is that instead of pseudo random bits, the bitsproviding the maximum additional information are used as redundancybits. This is iteratively repeated until the threshold of the requiredMutual Information for error free decoding is reached. The algorithm forthe calculation of the redundancy data bits in the server based on thereceivers CSI in pseudo code is the following:

  RoD bits to request = 0 while (current MI < required MI) {  for (allbits in FEC codeword) {   find bit with maximum additional MI (by meansof LUT)  }  current MI = current MI + additional MI  RoD bits to request= RoD bits to request + 1; }

In short: The algorithm describes a method to estimate the number ofretransmitted bits for error free decoding, based on the channel stateinformation, with optimum performance, but high computationalcomplexity.

Thus, in such an embodiment the redundancy calculator 38 is configuredto add additional Mutual Information resulting in the receiver, when aparticular bit is known, to available Mutual Information for each bitlocation of a particular codeword until a threshold of an overall MutualInformation required for correct decoding is reached and to determinethe required amount of redundancy data required for correct decoding ofsaid particular codeword based thereon. Preferably, said redundancycalculator 38 is configured to determine said additional MutualInformation for several or all bits of a channel symbol. Also in such anembodiment the receiver 3 d preferably comprises a storage 40 thatstores said additional Mutual Information for a plurality of codes, inparticular for a plurality of code rates and/or codeword lengths.

Accordingly, in such an embodiment the redundancy calculator 42 isconfigured to add additional Mutual Information resulting in thereceiver, when a particular bit is known, to available MutualInformation for each bit location of a particular codeword until athreshold of an overall Mutual Information required for correct decodingis reached and to determine the required amount of redundancy datarequired for correct decoding of said particular codeword based thereon.Further, preferably the redundancy calculator 42 is configured todetermine said additional Mutual Information for several or all bits ofa channel symbol. Still further, preferably a storage 44 is providedthat stores said additional Mutual Information for a plurality of codes,in particular for a plurality of code rates and/or codeword lengths. Instill another embodiment the redundancy calculator 42 is configured touse bits of a channel symbol providing the maximum additional MutualInformation, resulting in the receiver, when a particular bit is known,as redundancy data. In a similar way, the receiver could, using channelstate information, determine the required number of bits of a channelsymbol providing the maximum additional Mutual Information.

The broadband server then transmits the redundancy data bits to thereceiver via broadband, which is then able to calculate the positions ofthe redundancy data bit within the FEC codeword with the same algorithmthat has been used in the redundancy data server to generate the bits.The receiver is then able to recombine and decode the FEC codeword.

To reduce the number of comparisons to find the optimal bit from theLUT, the LSBs only can be transmitted first, then the bits at bitposition LSB-1 within the QAM symbol, and so on, since these bits have ahigh probability to carry the lowest Mutual Information. This isneglected in the pseudo code for simplicity.

In principle it is also possible to determine the number of requiredbits based on other parameters like SNR or MER. However, SNR and MER donot allow for such an accurate estimation taking the CSI into account.Rough numbers for the required RoD amount must be stored in the serverand receiver that have been determined by simulation for different deltaSNR values (required SNR—actual SNR). That is, the estimation of therequired number of redundancy data bits based on SNR or MER is lessaccurate compared to the Mutual Information and therefore not wellsuited here.

In the following synchronization between receiver and broadband serverwill be explained.

The signaling about how the received data must be combined in thereceiver generally takes place in the broadband network. As a result theframe structure employed in the broadcast network does not necessarilyneed any extension. However, at the physical layer an identification ofthe FEC-encoded data segments is required for the synchronizationbetween the data from both terrestrial and broadband networks. Besides,at the application layer the redundancy data can be signaled as anadditional service, therefore a linkage to the respective originalservice shall be given.

Current terrestrial broadcasting systems like DVB-T or DVB-T2 contain nosuitable mechanism for a unique identification of FEC packets/BBFrames,although the available timestamp of DVB-T2 (ISSY counter) might beapplicable to some extent. However the limited time range of the ISSYcounter might prevent a reliable packet identification An unambiguousmechanism is therefore required to inform the broadband server, whichBBFrames could not be correctly decoded. One solution would e.g. be acounter related to each FEC packet, whose value is increased after eachFEC packet, allowing for the unique identification of a FEC packet. Ifit is intended to introduce the concept of using redundancy (on demand;RoD) in broadcasting systems without such unique packet identificationalternative approaches need to be used. A specific amount ofsoft-information (LLR) values of the LDPC or BCH parity block (in caseof DVB-T2) of the erroneous packet can be used as a fingerprint thatidentifies the packet. This is possible, since even a slight differencein the payload between packets leads to different parity blocks. Basedon the sequence of LLR values, the broadband server can perform acorrelation to achieve the synchronization. This allows for thesynchronization between broadband server and the receiver even if therequired SNR in the receiver is too low to decode any FEC packetscorrectly.

Thus, in an embodiment of a broadband server 4 c depicted in FIG. 16 itcomprises, in addition to the elements of the broadband server 4 b shownin FIG. 14, an identification unit 45 that identifies a data packet forwhich redundancy data shall be determined by use of a correlation usingsoft information values of data of said data packet, in particular ofparity data, contained in a request received from a receiver, whereinsaid redundancy calculator 42 is configured to use the information aboutthe identity of the data packet to determine redundancy data for saiddata packet. In the same way, the receiver could identify the datapacket to which received redundancy data belong based on the correlationusing soft information of the demodulator and/or the decoder.

If the receiver was already able to decode some FEC packets, thetransmission of soft-information for correlation in the broadband serveris not necessary, since a subset of the parity blocks of correctlydecoded preceding FEC packets can be used for packet identification. Insuch cases the known and hard decided fingerprint of the last correctlyreceived packet and the number of erroneous packets is transmitted tothe broadband server, which then sends the required amount of redundancyfor the requested packets. For this identification approach even a smallamount of bits is sufficient, since no correlation in the receiver isrequired. It must only be assured that the fingerprint uniquelyidentifies the FEC packet. Assuming that the parity blocks are binarysequences with equal distribution, the probability that a fingerprintsequence with length n is not unique for m preceding FEC packets is

$p = {1 - {\prod\limits_{k = 1}^{m}\;{\left( {\frac{1}{2^{n}}\left( {2^{n} - k + 1} \right)} \right).}}}$

Based on this formula the required number of bits can easily becalculated for a given maximum misdetection probability p and the numberof FEC packets the identification is carried out with. The misdetectionprobability p is given in the following table for exemplary values of mand n. It becomes clear that increasing the fingerprint length m,decreases the probability for misdetection.

m\n 8 16 24 32 40 48 56 64 2 3.91E−03 1.53E−05 5.96E−08 2.33E−109.09E−13 3.55E−15 1.39E−17 5.42E−20 10 1.63E−01 0.00068644 2.68E−061.05E−08 4.09E−11 1.60E−13 6.25E−16 2.44E−18 25 0.702147 0.004567741.79E−05 6.98E−08 2.73E−10 1.07E−12 4.16E−15 1.63E−17 100 1 0.07278450.000295 1.15E−06 4.50E−09 1.76E−11 6.87E−14 2.68E−16 250 1 0.3784470.00185348 7.25E−06 2.83E−08 1.11E−10 4.32E−13 1.69E−15 1000 1 0.9995290.0293343 0.000116292 4.54E−07 1.77E−09 6.93E−12 2.71E−14 2500 1 10.169892 0.00072704 2.84E−06 1.11E−08 4.34E−11 1.69E−13 10000 1 10.949234 0.0115729 4.55E−05 1.78E−07 6.94E−10 2.71E−12However, the success rate can be further increased if the frame numberand the number of the FEC block within the frame are transmitted.

Thus, in such an embodiment of a broadband server 4 c depicted in FIG.16 the identification unit 45 identifies a data packet for whichredundancy data shall be determined by use of a number of bits, inparticular parity bits, of the last correctly decoded codewordscontained in a request received from a receiver. Further, the redundancycalculator 42 is configured to use the information about the identity ofthe data packet to determine redundancy data for said data packet. Inanother embodiment a packet counter or a timestamp (ISSY) can be usedfor packet identification.

In the following cooperative decoding with distributed receivers will beexplained.

Most TV devices nowadays are integrated with terrestrial broadcastreceiver. But the TV devices work alone with the locally receivedsignals. However, as home networks are being installed in more and morehousehold, the receivers can be connected to each other as well, so thata cooperative decoding of the broadcast signal becomes realizable. Aspace-diversity will be created when the receivers can carry out thedecoding jointly and thus an improvement of the signal quality will alsobe possible. This concept is operating without a server that has perfectknowledge of the transmitted signal. Instead the redundancy data isgenerated in a cooperative fashion.

In the embodiment of a broadcast system shown in FIG. 17, n receiversRx1, Rx2, . . . , R are connected via a server, which may be locatedseparately or together with one of the receivers. After receiving thebroadcast signal each receiver checks whether and where (temporally orspectrally) redundancy data is required and makes a request to theserver. Having the requests from each receiver, the server requires thenecessary data from each receiver, encode it, and distribute it to thereceivers that need it.

Assuming an example with two receivers, Rx1 and Rx2, three cases mayhappen to each signal part (temporally or spectrally).

1. Both receivers can decode it correctly by themselves. No dataexchange is needed in this case.

2. Both receivers cannot decode it correctly by themselves. The LLRs ofthe signal are quantized and transmitted to the server, who adds themtogether and multicasts the signal to both receivers. Afterwards thereceivers precede the decoding process with help of the received LLRs.3. One receiver can decode it correctly by itself while the other not.In best case, another signal part can be found where the situation isreversed and the LLRs of these two signal parts can be added andforwarded by the server. Then each receiver can achieve the desired partby subtracting their own signal (network coding alike).

For example:

Rx1 send S2 and Rx2 send S1′ to the server. The server transmitted thenS2+S1 back to each receiver, which can get the desired signal withsubtraction.

Such a broadcast system thus comprises a broadcast transmitter thatbroadcasts broadcast signals in a coverage area for reception byterminals comprising a broadcast receiver and a broadband receiver, abroadband server that provides redundancy data to terminals within saidcoverage area, and one or more terminals comprising a broadcast receiverand a broadband receiver, wherein said broadband server is configured toobtain redundancy data required by a terminal from one or more otherterminals.

The information exchange can take place autonomously when the receiversare somehow connected to each other (e.g. by a home network viaEthernet) such that a distributed network is resulting. This approach isshown in FIG. 18. In this case, the server is not necessary and datarequest, coding and flow control are controlled by receivers themselves.Such a broadcast system thus comprises a broadcast transmitter thatbroadcasts broadcast signals in a coverage area for reception byterminals comprising a broadcast receiver and a broadband receiver, andone or more terminals comprising a broadcast receiver, wherein saidterminals are configured to obtain redundancy data required by aterminal from one or more other terminals.

Obviously, numerous modifications and variations of the presentdisclosure are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

In so far as embodiments of the invention have been described as beingimplemented, at least in part, by software-controlled data processingapparatus, it will be appreciated that a non-transitory machine-readablemedium carrying such software, such as an optical disk, a magnetic disk,semiconductor memory or the like, is also considered to represent anembodiment of the present invention. Further, such a software may alsobe distributed in other forms, such as via the Internet or other wiredor wireless telecommunication systems.

The elements of the claimed devices and apparatus may be implemented bycorresponding hardware and/or software elements, for instanceappropriated circuits. A circuit is a structural assemblage ofelectronic components including conventional circuit elements,integrated circuits including application specific integrated circuits,standard integrated circuits, application specific standard products,and field programmable gate arrays. Further a circuit includes centralprocessing units, graphics processing units, and microprocessors whichare programmed or configured according to software code. A circuit doesnot include pure software, although a circuit includes theabove-described hardware executing software.

Any reference signs in the claims should not be construed as limitingthe scope.

The invention claimed is:
 1. A receiver device for receiving data,comprising: a first receiver configured to receive an input data streamreceived from an Orthogonal Frequency Division Multiplexing (OFDM)broadcast, via a first input interface, the input data stream comprisinga plurality of channel symbols represented by Quadrature AmplitudeModulation (QAM) constellation points in a constellation diagram from afirst network; a demodulator configured to demodulate said channelsymbols into codewords; a decoder configured to attempt Forward ErrorCorrection (FEC) decoding of the codewords and to determine success ofthe FEC decoding and, if success is determined, to output decoded data,the FEC decoding including a Low Density Parity Check (LDPC) decoding; asecond receiver configured to obtain redundancy data via a second inputinterface from a second network which is different to the first network,if the decoder is unsuccessful in performing FEC decoding; wherein thedecoder is further configured to determine the positions of theredundancy data with respect the codewords and output decoded data froma combination of the redundancy data and codewords derived from theinput data stream.
 2. The receiver device as claimed in claim 1, whereinthe decoder is configured to attempt Forward Error Correction (FEC)decoding of the codewords using further information associated with thecodeword.
 3. The receiver device as claimed in claim 2, wherein thefurther information associated with the codeword is received via thefirst receiver.
 4. The receiver device as claimed in claim 2, whereinthe further information associated with the codeword is received via thefirst receiver as broadcast information.
 5. The receiver device asclaimed in claim 1, wherein the first receiver is a broadcast receiverand the second receiver is a broadband receiver.
 6. The receiver deviceas claimed in claim 1, wherein the redundancy data is signaled to thereceiver device as an application layer service.
 7. The receiver deviceas claimed in claim 1, wherein the second receiver is configured tofirst request specific redundancy data before obtaining the redundancydata.
 8. The receiver device as claimed in claim 7, wherein theredundancy data requested corresponds to specific bit positions in theinput data stream.
 9. The receiver device as claimed in claim 7, whereinthe redundancy data requested corresponds to specific positions amongstthe codewords.
 10. The receiver device as claimed in claim 7, whereinthe second receiver is configured to request an amount of redundancydata required to successfully decode codewords demodulated by thedemodulator.
 11. The receiver device as claimed in claim 7, wherein thesecond receiver is configured to request an amount of redundancy datarequired to successfully decode the input data stream, the input datastream having been broadcast to the receiver device.
 12. The receiverdevice as claimed in claim 1, wherein the decoder is configured todetermine the positions of the redundancy data as bits with respect tothe codewords.
 13. The receiver device as claimed in claim 1, whereinthe decoder is configured to determine the positions of the redundancydata as bits within a codeword.
 14. The receiver device as claimed inclaim 1, wherein the redundancy data is received as unicast data.
 15. Aterrestrial television device comprising the receiver device as claimedin claim
 1. 16. A method comprising: receiving an input data streamreceived from an Orthogonal Frequency Division Multiplexing (OFDM)broadcast, via a first input interface, the input data stream comprisinga plurality of channel symbols represented by Quadrature AmplitudeModulation (QAM) constellation points in a constellation diagram from afirst network; demodulating said channel symbols into codewords;attempting Forward Error Correction (FEC) decoding of the codewords anddetermining success of the FEC decoding and, if success is determined,outputting decoded data, the FEC decoding including a Low Density ParityCheck (LDPC) decoding; obtaining redundancy data via a second inputinterface from a second network which is different to the first network,if FEC decoding is unsuccessful; and determining the positions of theredundancy data with respect the codewords and output decoded data froma combination of the redundancy data and codewords derived from theinput data stream.
 17. The method as claimed in claim 16, comprisingattempting Forward Error Correction (FEC) decoding of the codewordsusing further information associated with the codeword.
 18. The methodas claimed in claim 16, comprising receiving the input data stream viabroadcast and obtaining the redundancy data via broadband.
 19. Themethod as claimed in claim 16, comprising receiving signallinginformation indicating the redundancy information is an applicationlayer service.
 20. The method as claimed in claim 16, comprising requestan amount of redundancy data required to successfully decode the inputdata stream received via broadcast.
 21. The method as claimed in claim16, comprising determining the positions of the redundancy data as bitswith respect to the codewords.